STATCOM (Static Compensator) converters based on chain-link topology are emerging technology nowadays. A three-phase chain-link converter, being a voltage source converter (VSC) device, comprises three phase legs, each phase leg comprising a number of series-connected cell converters. FIG. 1 illustrates such a three-phase chain-link converter 1 comprising a voltage source converter 2. The three phase legs are denoted L1, L2, L3 and are connected in a delta arrangement, but are in other set-ups connected in a wye arrangement. Each phase leg comprises a number of series-connected converter cells, e.g. phase leg L1 comprises converter cells 10, 11, 12, . . . , n. Each converter cell 10, 11, 12, . . . , n, sometimes denoted converter link, comprises four valves connected in a H-bridge arrangement with a capacitor unit 13. Each valve comprises a transistor switch 19, such as an insulated gate bipolar transistor (IGBT) having a free-wheeling diode 20 (also denoted anti-parallel diode) connected in parallel thereto.
The chain-link converter 1, and in particular each phase leg thereof, is connected to an electrical power grid 14, in the illustrated case a three-phase network, via a phase reactor 15. Further, each phase leg is connected to the power grid 14 via a starting resistor 17 connected in parallel with a switch 16 and via an AC circuit breaker 18.
Generally, when the chain-link converter of this type is to be energized and started, and using phase leg L1 as an example, the circuit breaker 18 is switched so as to provide a current path from the power grid 14, through the starting resistor 17, the phase reactor 15 and through the diodes of the chain-link converter 1, thereby charging the capacitor 13. When the capacitor voltage has reached a predetermined level, the starting resistor 17 is short-circuited by closing the parallel connected switch 16. The capacitor voltage will then increase further and the switches become controllable.
Start-up of such chain-link converters involves a number of considerations. For example, in WO 2010/051854 one aspect of start-up of converters is addressed. In this publication a method for starting a chain-link converter is disclosed, wherein a power source is connected to at least one converter cell of a phase leg, thereby improving the start-up e.g. by lessening the stress put on components such as diodes.
In another aspect, during start-up of the chain-link converter, some of the electronic switches may be defective, they could for example being in open circuit or in short-circuit. If energizing the chain-link converter 1 by limiting in-rush currents by means of the phase reactor 15 only, failed electronic switches in open circuit may be exposed to a voltage level above their voltage withstand capability and thus cause other electronic switches to break down.
In view of the above, it would be desirable to provide start-up procedures of chain-link converters for avoiding cascading failure of other components.